In many antenna systems, the ability to control mutual coupling between elements in an antenna array or between separate arrays is often of critical importance. As many antenna systems need to be operable over a broad frequency range, such coupling of elements must also be maintained over a broad frequency range to meet design objectives. Undesired coupling can cause performance degradation or even damage to the underlying hardware of an antenna system. To mitigate this problem, a compact structure which can be utilized to control surface wave propagation and reduce coupling between elements is usually needed.
The most common method used for reducing the propagation of surface waves, so that undesired coupling can be avoided is the use of electromagnetic band gap (EBG) structures in between the elements of interest. EBG structures are typically designed as compact and conformal structures made of periodic metal and dielectric structures to provide a frequency range in which no surface waves can propagate, this effectively puts a gap in the band of allowed surface wave modes. The creation of the band gap caused by the EBG structure acts to essentially eliminate coupling.
One of the earliest example of an EBG structures consisted of a doubly periodic array of simple metallic patches connected to a ground plane by vias (the unit cells of the EBG are sometimes referred to as mushroom-type or mushroom-like structures). In a limited frequency range, this structure has a very high impedance.
Due to the bandwidth limitation of this structure, different methods have been proposed to improve the frequency range over which EBG performance can be realized. The primary method that has been used to improve bandwidth is cascading multiple EBG structures, each targeting a different frequency band. Each section of the cascaded structure is composed of differently sized unit cells that correspond to a different target frequency range. If the unit cell sizes are strategically chosen, their respective bands can be designed to overlap with one another to form a single device with a broader bandwidth of operation. However, use of multiple cascaded sections requires a system that is very large due to the need for a sub-structure to cover each frequency range. Compact structures are typically not available for designs using such an EBG structure. Since space available for an antenna system is often limited, the inability to utilize a compact structure is often a negative feature for such EBG designs.
Another method that has been used is to modify the path to ground from the mushroom structure in each section. This effectively changes the inductance and therefore changes the resonant frequency of the circuit, resulting in similar control over the frequency range of operation as previously described with respect to the cascaded section EBG design. Because it is difficult to increase the inductance of this basic structure, it is necessary to lower the inductance. This leads to the modified frequency range being at a higher frequency than the original structure, therefore the size of the structure relative to wavelength is not reduced at the lower end of the frequency band.
A new EBG structure is needed that permits the use of a compact structure that also provides improved bandwidth. Preferably, such a new structure permits use of a design methodology that makes it possible to relatively quickly design a desired EBG structure that meets design objectives for an antenna system by circumventing of a need to perform a full wave simulation each time an EBG surface is changed or configured during the design of the structure.